Current blow-molding machinery is designed to manufacture hollow plastic containers by extruding a parison between opposed, parted mold halves. The mold is then closed and air is injected into the parison, blowing and distending it outward so that it conforms itself to the inner contours of the mold which match the outer shape of the bottle to be produced. The newly-formed container is allowed to harden, then is extracted or dropped from the mold.
Some plastic bottle manufacturers produce blow-molded articles by using rotary blow-molding machines. A typical rotary blow-molding machine includes a rotating carrousel that carries two or more mold stations around a circular mold station path. Each mold station includes a mold with a mold cavity. The carrousel is indexed to hold each mold station momentarily motionless at each of a series of blow molding "work stations". For the purposes of this application a "work station" will be defined as a position where each mold station must pause in its circular path for a particular operation to be carried out. A "work station series" will be defined as a progression of work stations necessary to complete a blow molding process on a single mold station in a single carrousel revolution. Blow molding operations are carried out sequentially on each mold as each mold station rotates through a work station series, pausing momentarily at each work station in the series.
Rotary blow-molding machines may, of course, include any number of mold and work stations, and may include more than one work station series arranged around a single mold station path. In rotary blow-molding machines with a single work station series, there is a simple relationship that determines the minimum number of carrousel indexing positions: The carrousel must pause in a number of indexing positions that is at least equal to the number of work stations in the series and at least equal to the number of mold stations. The reason for this is that each mold station must pause at each work station in the series one time during each carrousel rotation to complete the blow molding process at each mold station. As long as the number of mold stations is equal to or greater than the number of blow-molding work stations where each mold station must pause, then the carrousel must index at least as many times as there are mold stations. If, for example, there are only two mold stations spaced 180 degrees apart, and only one work station where each mold station must pause, the carrousel must index 180 degrees twice during each rotation to make each mold station stop at the work station one time.
In many rotary blow molding machines the only blow-molding operations that require a mold station to pause at a fixed work station are the parison-extrusion and article-release operations. However, following the extrusion station and preceding the release station the process requires that there be sufficient "blow time" and cooling time for each blow-molded article to properly form and harden in the cavity within each mold.
To provide sufficient "blow time" and cooling time, designers usually position the release station to immediately precede the extrusion station. This causes each mold and mold station to travel a large angular distance around the mold station path from the extrusion station to the release station, and a smaller angular distance from the release station back to the extrusion station. The average carrousel angular velocity must be slow enough to allow each mold sufficient "blow time" and cooling time as it travels this angular distance.
By adding more mold stations and indexing pauses to a carrousel, designers are able to minimize the impact that these cooling and "blow time" requirements have on the total average time it takes to produce a blow-molded bottle. While cooling and blow-time requirements must generally remain the same, additional mold stations and indexing pauses allow more extrusion and release operations to occur per revolution. However, carrousel space limitations and the absence of a work station in the typically small angular distance between the release station and the extrusion station make it difficult to add another operation, such as in-mold labeling, to the blow-molding process.
Current rotary blow-mold carrousels are generally designed to index a number of times equal to the number of mold stations supported on the carrousel. For example, U.S. Pat. Nos. 3,854,855, 3,936,521 and 3,941,863 to Pollock et al., each disclose six-mold-station carrousels that index six times per revolution; U.S. Pat. No. 4,439,127 to Frohn discloses a five-mold-station carrousel that indexes five times per revolution; and U.S. Pat. No. 4,233,019 to Sawa et al., U.S. Pat. No. 4,834,643 to Klinedinst et al. and Japanese Pat. No. 402,165,922A to Calsonic et al. each disclose four-mold-station carrousels that index four times per revolution.
The reason that engineers design rotary blowmolder carrousels to index a number of times equal to the number of mold stations is because the carrousel must index at least this number of times to cause each mold station to pause at each work station. However, as designers increase the number of mold stations in a rotary blow-molder, they must either enlarge the carrousels to accommodate the larger number of mold stations, or must make the mold stations smaller--so that more can fit on the same size carrousel.
Moreover, for designers to add additional operations such as in-mold labeling to rotary blow molding machines with, as is found in the prior art, the same number of indexing positions as mold stations, they must displace the article take-out work station with the new in-mold labeling apparatus. This requires that the take-out work station be re-located to the next preceding work station along the mold station path. This, in turn, compresses all other blow molding operations, including cooling and blowing, into a smaller portion of the mold station path. This compressions would require the carrousel to either be larger or to rotate more slowly to allow sufficient cooling and blow time. It would also constitute an inefficient use of the latter portion of the mold station path between the relocated take-out station and the extrusion station.
What is needed is a rotary blowmolding apparatus that allows additional operations to be performed on each mold between the release and extrusion stations without increasing the number of mold stations mounted on the carrousel, without compressing blow molding operations into the early portion of the mold station path following parison extrusion, and while maximizing the portion of the mold station path used for blowing and cooling.